1. Technical Field
The present invention relates to power measurements for linearly modulated radio frequency signals and more particularly to a method and a device for compensating a data-dependency of a power measurement caused by linear modulation.
2. Discussion of the Prior Art
The condition of radio systems such as mobile phone systems or satellite systems has to be constantly monitored by power measurements to ensure a high quality of transmission and to avoid damages occurring at output stages that resulting from excessively high power levels.
Within current transceiver units in the Global System for Mobile Communications (GSM) base transceiver stations a measurement of the output power (Pfwd) transmitted via an output port and a separate measurement of the power, which is reflected (Prefl) at the output port, are performed. Pfwd is e.g. measured at the beginning of the useful part of a transmitted Gaussian Minim Shift Keying (GMSK) burst and Prefl is e.g. measured at the end of the useful part of the burst. The power measurements are thus performed time multiplexed.
Based on the measured values of Pfwd and Prefl, the condition of the base transceiver station can be assessed, e.g. by calculating the voltage standing wave ratio (VSWR). In W094/24576 a method for supervising the condition of a transmitter antenna of a radio system based on the VSWR is described. The VSWR is a measurement of impedance mismatch between a transmission line and its load. The higher the ratio, the greater the mismatch. In other words, as the VSWR increases, the transmission quality declines.
The GMSK modulation technique is a non-linear modulation technique which introduces only very small variations of the radio frequency signal envelope during the useful part of the burst. The powers Pfwd and Prefl are not data dependent and thus can be measured at arbitrary instances in time during a burst. The measurement results can be used directly for the calculation of a the matching VSWR.
Linear modulation techniques insert large variations in output power depending on the sequence of data which are transmitted. The dynamic power range spreads up to 19 dB over the useful part of the burst, as shown in FIG. 5. The measurement of Pfwd and Prefl during the useful part of the burst does not lead to reliable results for linear modulation technique, due to the data dependency of the output power. The maximum variation in average output power due to linear modulation between the first and the second half of a single burst can attain 2 dB if every symbol is measured and used for calculating the average output power. If less symbols are measured, the maximum variation in average output power might even increase.
These data-dependent fluctuations of the average output power within a single burst may lead to serious problems when assessing the condition of a radio system based on time-multiplexed measurements. If, e.g., the real VSWR and the calculated VSWR differ by 2 dB or more due to the data-dependency of the output power, the system may e.g. conclude that an antenna feeder cable is disconnected and an alarm will erroneously be sent. The inaccuracy of power measurements resulting from the date-dependency of the output power can thus lead to misinterpretations of the result of the VSWR calculation. Even more severe problems can result from misinterpretations of measurements of Pfwd since the measured value of Pfwd is an important parameter for the correct supervision of the output power in order to avoid damages resulting from excessively high output power levels.
There is, therefore, a need for a method and device for compensating the data-dependency of time multiplexed measurements of Pfwd and Prefl for linear modulation techniques.